Optical materials are characterized by their refractive index. Whenever light travels from one material to another of different index, some of the light is reflected. Unwanted reflections can be substantially reduced by providing an anti-reflective coating. Anti-reflective (AR) coatings (also called antireflection coatings) are a type of optical coating applied to the surface of optical devices to reduce reflection. Many coatings consist of transparent thin film structures with one or more alternating layers of contrasting refractive index. Layer thicknesses are chosen to produce destructive interference in the beams reflected from the interfaces, and constructive interference in the corresponding transmitted beams. For a single low refractive index thin film coating on an optical article with refractive index n, in order to reach the maximum effectiveness, the anti-reflective coating should have an optical thickness (the physical thickness multiplied by its own refractive index) about a quarter of the wavelength of the incoming light and have a refractive index of the square root of n if the other interface is air. Most optical articles have a refractive index ranging from 1.4 to 1.6.
It is known that low refractive index anti-reflective coatings can be prepared from fluorinated polymers. The refractive index of a fluorinated polymer correlates with the amount of fluorine in the polymer. Increasing the fluorine content in the polymer decreases the refractive index of the polymer. Considerable industry attention has been directed towards anti-reflective coatings containing fluorinated polymers.
Fluoropolymers with low crystallinity that are soluble in organic solvents typically form coatings having undesirable mechanical properties, such as poor abrasion resistance and poor interfacial adhesion between the fluoropolymer coating and the underlying optical display substrates such as plastics and glass. Various modifications have been explored in order to improve their abrasion resistance and adhesion to substrates.
There is a continuing need in the industry, in the field of optical displays, for anti-reflective coatings having low visible light reflectivity as well as good adhesion to optical display substrates and good scratch resistance.